Semiconductor ceramic materials for use in positive temperature coefficient thermistors have PTC characteristics such that the resistances thereof sharply increase when the semiconductor ceramic materials are heated to predetermined temperatures (Curie points). Known examples of such semiconductor ceramic materials having PTC characteristics include BaTiO3 materials.
The semiconductor ceramic materials need to be usable at elevated temperatures for applications such as thermistors for heaters. The following material is known: a (Ba, Pb)TiO3 semiconductor ceramic material, prepared by partly replacing Ba in BaTiO3 with Pb, having a high Curie point.
However, there is a problem in that a material of which the Curie point is increased by the addition of Pb as described above has a small temperature coefficient of resistance and the temperature coefficient of resistance thereof is voltage-dependent, as described in Japanese Unexamined Patent Application Publication No. 56-169301 (Patent Document 1). Therefore, a semiconductor ceramic material which contains no Pb, which has a high Curie point, and which is suitable for use in positive temperature coefficient thermistors is desired.
For example, Patent Document 1 and Japanese Unexamined Patent Application Publication No. 2005-255493 (Patent Document 2) disclose a (Ba, Na, Bi, Ln)TiO3 semiconductor ceramic material (Ln is a rare-earth element) for use in positive temperature coefficient thermistors.
However, an increase in Curie point tends to cause an increase in electrical resistivity. Therefore, compositions disclosed in Patent Document 1 or 2 have a Curie point of 120° C. or higher and an electrical resistivity of about 70 Ω·cm, which is relatively large. Therefore, compositions having lower resistivity are desired for use in thermistors for heaters.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 56-169301    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-255493